With the rapid development of the Internet and mobile communication, optical modules (mainly composed of transmitters, receivers, optical fibers, packages, circuit boards and auxiliary components) have become the core devices of optical fiber communication because of their high port density, small size, low power consumption and other characteristics, and are favored by users in the communication field. At present, various emerging fields are constantly emerging, and the demand for timely and stable data transmission is growing exponentially. Therefore, more efficient, reliable, and high-speed data transmission optoelectronic devices are needed to meet the development needs of the industry. High speed optical modules are optoelectronic devices that meet this demand. They are mainly used for high-speed data transmission, data centers, and computers, and are widely used in communication, computer, data center, and scientific research fields such as astronomy, particle physics, and life sciences.

When the high-speed optical module printed circuit board (PCB) is combined with the optical driver chip, wire bonding (also known as wire bonding, bonding, wire bonding) is usually used, which refers to the use of metal wires (gold wire, aluminum wire, etc.) for bonding, and then the use of hot pressing or ultrasonic energy to complete the connection of the internal interconnection wiring of the solid-state circuit in microelectronic devices, that is, the connection between the optical driver chip and the high-speed optical module printed circuit board or lead frame. During the process of gold wire bonding, there may be a phenomenon where the reliability of gold wire bonding fails due to printed circuit board reasons.

The current problems and main factors affecting wire bonding

For high-speed optical module bonding products, the WireBonding process is generally carried out after surface mounting, which means that other functional components such as resistors, capacitors, and small integrations on the PCB are mounted and then bonded. Once bonding failure occurs, the optical driver chip cannot be implanted, and the entire PCB will be unusable or even scrapped. At this time, not only the processing cost of the PCB, but also the mounted resistors, capacitors, and other components will face scrapping, resulting in high material costs.

Analyze the WireBonding requirements of high-speed optical modules and extract the main factors that cause WireBonding failure, as shown in Table 1. From the main factors affecting bonding, the reasons for bonding failure include PCB board PAD size, PAD coating thickness, surface cleanliness, bonding parameters, bonding methods, and other factors.

Overview of plasma cleaning application for optical module packaging

Plasma cleaning has become a standard configuration in chip packaging technology, and optical module products pursue both density and packaging volume, which is the same as the development path of the semiconductor industry. At the same time as the volume is reduced, the packaging form will impose higher requirements on the welding process of the factory. After plasma cleaning, the adhesion strength of the chip on the substrate is higher, and the bonding strength between the product pads and the gold wire is greater, which greatly improves the production efficiency and excellent rate of the product.

Plasma cleaning is the process of ionizing the working gas to produce plasma using a radio frequency power supply in a certain vacuum environment. Plasma contains a large number of active particles such as high-energy electrons, ions, and free radicals. In optical module packaging, these active particles undergo physical and chemical reactions with the surface of the optical module to be packaged, which can remove surface pollutants and oxide layers, and improve surface wetting performance.

The following optical module packaging products all use plasma cleaning technology:

Plasma cleaning effect

Contact angle test

The contact angle (droplet angle) test is an important testing method for reflecting surface cleaning conditions. The higher the angle, the smaller the surface force, which prevents the droplet from flowing. The smaller the angle, the stronger the surface force, and the droplet can fully adhere. The cleaned product shows a significant improvement in its surface adhesion. The cleanliness of the bonding surface can be determined by its surface wettability and hydrophilicity.

Changes in contact angle before and after plasma cleaning of customer samples

Push-pull force test

The push-pull force test can be used to check the strength of the wire. The product after plasma cleaning can completely avoid the occurrence of weak welding and other situations. It can greatly improve the surface suction between solder wires and pads, enhance product reliability and stability.

Performing plasma cleaning before gold wire bonding (wire bonding) in optical module packaging can effectively clean the bonding area, improve the surface energy of the bonding area to enhance bonding strength and consistency, and ensure the long-term reliability of the product.